Simon Cheng was held for 15 days for violating public security management regulations, police in Shenzhen, across the border from Hong Kong, said on Saturday.
"We welcome the release of Simon Cheng and are delighted that he can be reunited with his family. We will continue to provide support to them," a spokesman for Britain's foreign ministry said.
"Simon and his family have requested privacy and we would be grateful if this is respected."